Circuit for a gas-discharge lamp

ABSTRACT

The invention relates to a circuit for a discharge lamp, comprising: a first sub-circuit for connecting to mains voltage of a predetermined frequency for rectifying the mains voltage; a second sub-circuit connected to the first sub-circuit for providing an alternating current, preferably a square-wave current, required for the lamp; and a control circuit which is connected to the first and the second sub-circuit and which controls the frequency of the alternating current subject to a varying component of the mains voltage rectified by the first sub-circuit.

The present invention relates to a lamp circuit. Such circuits aremarketed by applicant with the purpose of making gas-discharge lamps, inparticular fluorescent lamps or metal-halide lamps, function in acorrect manner. Metal-halide lamps require for instance a relativelyhigh ignition voltage (about 3500-5000 V), whereafter during the stageof normal operation reached via one or more intermediate stages, thecircuit must supply a current that is as constant as possible to themetal-halide lamps at an operating voltage of for instance 100 V. Such alamp has a negative impedance characteristic on a short time scale,therefore it must be current controlled.

Such lamps and associated circuits have now proved their market value,particularly in shopping centers, public buildings and the like, as aresult of their relatively high light output. However, the knowncircuits are in some respects complicated and voluminous, which isundesirable in terms of component and fabrication costs.

The present invention provides a circuit for a lamp, comprising:

-   -   a first sub-circuit for connecting to mains voltage of a        predetermined frequency for rectifying the mains voltage;    -   a second sub-circuit connected to the first sub-circuit for        providing an alternating current required for the lamp; and    -   a control circuit which is connected to the first and the second        sub-circuit and which controls the frequency of the alternating        current subject to a varying component of the mains voltage        rectified by the first sub-circuit.

By virtue of the present invention such a circuit can be embodied so asto be simpler and less voluminous since a buffer capacitor in such acircuit can be reduced considerably in size, for instance by a factor often. The currently available electrolyte capacitors, which as a resultof drying out may have a limited lifespan, can moreover be replaced bymore robust film capacitors with smaller capacitance values.

The first sub-circuit preferably comprises a so-called pre-conditionerwhich, generally speaking, comprises a filter with one or more coils andcapacitors, a rectifier circuit such as a bridge rectifier incombination with a boost converter or a flyback converter comprising an(electronic) switch, and a buffer capacitor that is coupled to theoutput terminals.

In a further preferred embodiment, the second sub-circuit comprises aconverter circuit for stabilizing direct current and a switching devicefor providing a square-wave current of a desired level of for instance±100 V for normal operation of the lamp.

In a further preferred embodiment, the control circuit is connected onone side to an (electronic) switch in the first sub-circuit and on theother side to one or more (electronic) switches in the switching devicepart, so that the phase and/or frequency of the lamp current controlledby the commutator is controlled subject to a varying component of forinstance 50 Hz or a multiple thereof (for the USA and Japan 60 Hz or amultiple thereof).

In a further preferred embodiment of the present invention, thefrequency of the alternating current provided by the second sub-circuitis made equal to the frequency of a varying component of the mainsvoltage rectified by the first sub-circuit.

In a further preferred embodiment of the present invention, the controlcircuit controls the phase of the alternating current provided by thesecond sub-circuit such that this is the same as the phase of a varyingcomponent of the rectified mains voltage provided by the firstsub-circuit.

These and other aspects of the invention are explained hereinbelow withreference to associated Figures, in which:

FIG. 1 shows a block diagram of a circuit for a lamp according to apreferred embodiment of the present invention;

FIG. 2 shows a diagram of a part of the circuit of FIG. 1 according to apreferred embodiment;

FIG. 3 shows a diagram of a part of the circuit of FIG. 1 according to afurther embodiment;

FIG. 4 shows a diagram of a part of the circuit of FIG. 1 according to afurther embodiment; and

FIG. 5 shows a graph with voltage and current values varying in time inthe circuits of FIGS. 1-4 as an elucidation of the operation thereof.

FIG. 1 shows a circuit for a lamp 1 comprising a pre-conditioner 2 forrectifying the supplied alternating voltage us and bringing it to adesired voltage level. Depending on the requirements made on the currentdrawn from the mains 3, concerning the power factor and the harmonicdistortion thereof, the component values of the pre-conditioner areadapted. Adding an interference filter 4 can help to meet theserequirements. Part of the pre-conditioner is an energy buffer 5 to whichthe subsequent sub-circuits are connected. Provided across the energybuffer 5 is the rectified voltage U_(DC) with an average value in theorder of 400 V, to which a stabilizer 6 is connected for stabilizingdirect current supplied to a commutator 7 in which an igniter 8 forigniting lamp 9 is arranged. Commutator 7 provides for instance asquare-wave current i_(c) to lamp 9, which is for instance ametal-halide lamp with an output of 80 lm/W and a lifespan in the orderof 10,000 hours, with u₉ being the voltage across the lamp. Such lamps,which use such an electronic circuit, are supplied by applicant and beartype designations MH and CDM in the power range of 35-150 W. Controlcircuit 10, which is connected to pre-conditioner 2, stabilizer 6 andcommutator 7, controls these sub-circuits so as to provide theabove-stated properties. The light level can be controlled (dimming) bythe externally available light level control 11.

FIG. 2 is a more detailed diagram of circuit 1, representing thepreferred embodiment of the invention, showing the mains voltage 3 towhich the circuit is connected, followed by an interference filter 4consisting of coils 13 and 14 and capacitor 15. Pre-conditioner 2comprises the double-sided rectifier 16 followed by coil 17 and FET 18.After opening of the FET, the coil 17 wants to maintain the currentacross said FET by generating a voltage, resulting, via the diode 19, ina direct voltage U_(DC) across the buffer capacitors 20 and 21 thatperform the task of buffer capacitor 5 in FIG. 1. At the connection 22between capacitors 20 and 21, the voltage value is (0.5 U_(DC)). In thepreferred embodiment as shown in FIG. 2, the stabilizer, commutator andbuffer capacitor are combined in one circuit 23, the so-calledhalf-bridge commutating forward (HBCF) 23. The HBCF comprises FETs 24and 25 that are alternately made conducting or non-conducting at adetermined frequency by control circuit 10, the voltage u_(22,26) acrossthe points 22 and 26 of the circuit being alternately plus or minus halfthe rectified voltage. Coil 27 is present to provide a stabilizedcurrent to the series connection of lamp 9 and coil 30. The coil 30 incombination with capacitor 31 filters the varying component of the loadcurrent provided by coil 27. Diodes 28 and 29 are arranged parallel tothe FETs 24 and 25 to release stored energy from coil 27. To ignite thelamp 9, an igniter (not shown) is coupled via a transformer connectionto coil 30 during starting of the lamp 9, which igniter co-acts withcapacitor 31.

A further possible embodiment of the invention concerns a full-bridgecommutating forward (FBCF) 36 as shown in FIG. 3, comprising four FETs24,25, 32 and 33. FET 24 and 33 are alternately made conducting ornon-conducting and FETs 25 and 32 are alternately made non-conducting orconducting, respectively. Buffer capacitor 5 is present in this case tosupply the rectified voltage, so that the entire rectified voltageU_(DC) is provided across points 22 and 26 of the circuit.

In a still further embodiment of the invention (FIG. 4), the stabilizerand commutator are implemented separately as the down converter 37 andthe fall-bridge commutator 38. The down converter comprises a FET 39,diode 40, coil 41 and capacitor 42, the FET 39 being turned on and offby the control circuit 10. The FETs 24 and 33 and the FETs 25 and 32 aremade alternately conducting or non-conducting respectively.

FIGS. 5A and 5B show the voltages at different locations in the circuit.The mains voltage is 220 V-240 V at a frequency of about 50 Hz-60 Hz(FIG. 5A). The rectified voltage U_(DC) (FIG. 5B) on the buffercapacitor has an average voltage level of for instance 400 V, with thevarying component present thereon having a fundamental frequency of 100Hz-120 Hz with a peak-to-peak value which depends on the capacitance ofthe buffer capacitor 20+21 (FIG. 2) or 5 (FIG. 3 or 4). This varyingcomponent has a peak-to-peak value of for instance 10 V at a buffercapacitance of 68 μF and of for instance 100 V at a buffer capacitanceof 6.8 μF if the load is a 70 W lamp.

When the HBCF circuit is used (FIG. 2), half the voltage U_(DC) isprovided across the series of coils 27 and 30 and lamp 9. The lampcurrent i₉ (FIG. 5C) is a square-wave current with a value of about 0.8A if loaded with a 70 W lamp. The voltage u₉ of about 100 V across thelamp 9 is shown in FIG. 5D. After the moment of commutation, i.e. themoment the current changes direction, there will however occur aso-called restart peak 51 during which the voltage across the lampassumes a higher value in the order of 150 V for a short time (FIG. 5D).If the supplied voltage, i.e. the available open circuit voltage, is nowlower than the voltage of this restart peak, the lamp will go out.

The control circuit controls the FETs such that the square-wave currentapplied to the lamp is synchronized with the voltage across the buffercapacitor, the square-wave current having a frequency which is equal tothe fundamental frequency of the varying component of supply voltageU_(DC) This means that at the moment of commutation the voltageavailable to the lamp will always equal (half) the mean value of thesupply voltage U_(DC), the Open Circuit Voltage (OCV), and isguaranteed, so that during the commutation the OCV will always have avalue greater than the necessary re-ignition voltage of the lamp. Thelamp circuit can be optimized by reducing the size of the energy buffer,as a result of which also the lifespan and the reliability of thecircuit and of the lamp are improved.

A circuit for a lamp according to the present invention is an optimizedcircuit as regards both the requirements made on the current drawn fromthe mains by using a pre-conditioner and as regards the open-circuitvoltage that is available to the gas discharge lamp. In addition, thecapacitance of the buffer capacitor can be reduced, causing the lifespanof the circuit and the lamp to be increased and the size of the circuitto be reduced. The lamp used will burn reliably since the open circuitvoltage available to the lamp is always greater than the voltagedemanded by the lamp.

The present invention is not limited to the above-described preferredembodiment thereof; the rights sought are defined however by thefollowing claims, within the scope of which many modifications can beenvisaged.

1. Circuit for a lamp, comprising: a first sub-circuit for connecting tomains voltage of a predetermined frequency for rectifying the mainsvoltage; a second sub-circuit connected to the first sub-circuit forproviding an alternating current required for the lamp; and a controlcircuit which is connected to the first and the second sub-circuit andwhich controls the frequency of the alternating current subject to avarying component of the mains voltage rectified by the firstsub-circuit.
 2. Circuit for a lamp as claimed in claim 1, wherein thefirst sub-circuit comprises a filter with one or more coils andcapacitors, a rectifier circuit, an (electronic) switch and a buffercapacitor that is coupled to its output terminals.
 3. Circuit for a lampas claimed in claim 1, wherein the second sub-circuit comprises aconverter circuit for stabilizing direct current and a switching devicefor providing a square-wave current of a desired level of for instance±0.8 A for normal operation of the lamp.
 4. Circuit for a lamp asclaimed in claim 1, wherein the control circuit is connected on one sideto an (electronic) switch in the first sub-circuit and on the other sideto one or more (electronic) switches in the switching device, so thatthe phase and/or frequency of the lamp current controlled by theswitching device is controlled subject to a varying component of forinstance 50 Hz or a multiple thereof (in the USA and Japan 60 Hz or amultiple thereof).
 5. Circuit for a lamp as claimed in claim 1, whereinthe frequency of the alternating current provided by the secondsub-circuit is made equal to a varying component of the mains voltagerectified by the first sub-circuit.
 6. Circuit for a lamp as claimed inclaim 1, wherein the control circuit controls the phase of thealternating current provided by the second sub-circuit such that this isthe same as the phase of a varying component of the rectified mainsvoltage supplied by the first sub-circuit.
 7. Circuit for a lamp asclaimed in claim 1, wherein the second sub-circuit comprises an igniterfor generating voltage pulses across the lamp so as to ignite the lamp.8. Circuit for a lamp as claimed in claim 1, wherein the rectified mainsvoltage is in the order of magnitude of 400 V and the voltage across thelamp is in the order of magnitude of 100 V to 150 V.
 9. Circuit for alamp as claimed in claim 1, wherein the varying component of therectified mains voltage has a peak-to-peak value in the order ofmagnitude of 10-100 V.
 10. Method for operating a lamp, comprising thesteps of: rectifying a supplied mains voltage and bringing it to adesired voltage level; and generating an alternating current; whereinthe frequency of the alternating current is controlled subject to avarying component of the rectified mains voltage.
 11. Method foroperating a lamp as claimed in claim 10, wherein the phase of thealternating current is equal to the phase of the varying component ofthe rectified mains voltage.